1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus such as a copier, a printer or a facsimile, and an image forming method and a program using the same.
2. Description of the Prior Art
Conventionally, in an electrophotographic image forming apparatus, a developer used for forming a toner image on a photoreceptor drum is largely divided into two types. For example, there are a developer using a mono-component toner and a dual-component developer including a nonmagnetic toner and a magnetic carrier.
Since a mono-component developing system is suitable for miniaturization but not suitable for high-speed development, most of image forming apparatuses attaining high-speed and long-life employ a dual-component developing device. In this type of the developing device using a dual-component developer, a carrier itself in the dual-component developer is not consumed and remains in the developing device, thus the carrier is not reduced. On the other hand, a toner is consumed in a development operation and is reduced. Hence, in order to prevent from destabilizing image quality caused by a decrease in the toner constituting the dual-component developer, toner density control is performed in which the toner is appropriately replenished to maintain a toner density of the dual-component developer within an appropriate range.
Generally, as a method for controlling a toner density, two kinds of controlling methods are often combined. One of the methods is such that a toner consumption is calculated according to a printing rate of an input image to replenish a toner. The other method is such that a density of a reference toner image formed on the surface of an electrostatic latent image carrier (photoreceptor drum) is detected to replenish a toner based on a given density value which is determined preliminarily and a comparison result.
As part of stabilization of a toner density of a developer, Patent Literature 1 (Japanese Patent Application Laid-open No. 2007-128010) proposes a technology for controlling a toner density by forming an image density patch for correction on a photoreceptor drum periodically, comparing a density value detected by a density sensor with a preliminarily set target value, and correcting a quantity of light of an exposure apparatus and a developing bias. Moreover, a technology for controlling a toner density by correcting a preliminarily set target value itself of an image density patch based on a relation between a pixel count cumulative value and a toner supply amount cumulative value is proposed.
In the control of a toner density described above, there are problems in that, when a pixel count value of an input image is accumulated and a toner replenishment amount is determined depending on the cumulative value, it is impossible to confirm that the toner amount to be supplied based on the calculated toner replenishment amount has been replenished correctly and it is also impossible to indicate the toner amount in a developer that has been actually consumed correctly. Thus, there is a problem that image quality of a printing image is deteriorated with the toner density out of the appropriate range.